RFC 3891

Network Working Group R. Mahy
Request for Comments: 3891 Cisco Systems, Inc.
Category: Standards Track B. Biggs
R. Dean
September 2004
The Session Initiation Protocol (SIP) "Replaces" Header
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document defines a new header for use with Session Initiation
Protocol (SIP) multi-party applications and call control. The
Replaces header is used to logically replace an existing SIP dialog
with a new SIP dialog. This primitive can be used to enable a
variety of features, for example: "Attended Transfer" and "Call
Pickup". Note that the definition of these example features is non-
normative.
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RFC 3891 The SIP "Replaces" Header September 2004
o A unique call-id may be given to the replacement call. This
avoids dialog matching problems in any of the related User Agents.
o There are no adverse effects if the header is unsupported.
The Replaces header enables services such as attended call transfer,
retrieve from park, and transition from locally mixed conferences to
two party calls in a distributed peer-to-peer way. This list of
services is not exhaustive. Although the Replaces header is
frequently used in combination with the REFER [8] method as used in a
Transfer [12], they may be used independently.
For example, Alice is talking to Bob from phone1. She transfers Bob
to a Parking Place while she goes to the lab. When she gets there
she retrieves the "parked" call from phone2 by sending an INVITE with
a Replaces header field to Bob with the dialog information Bob shared
with the Parking Place. Alice got this information using some out of
band mechanism. Perhaps she subscribed to this information from the
Parking Place (using the session dialog package [13]), or went to a
website and clicked on a URI. A short call flow for this example
follows. (Via and Max-Forwards headers are omitted for clarity.)
Alice Alice Parking
phone1 phone2 Bob Place
| | | |
|<===============================>| |
| | | |
| Alice transfers Bob to Parking Place |
| | | |
|------------REFER/200----------->| *1 *2 |
|<--NOTIFY/200 (trying)-----------|--INVITE/200/ACK-->|
|<--NOTIFY/200 (success)----------|<=================>|
|------------BYE/200------------->| |
| | | |
| | | |
| Alice later retrieves call from another phone |
| | | |
| *3 |-INV w/Replaces->| |
| |<--200-----------| |
| |---ACK---------->|----BYE/200------->|
| |<===============>| |
| | | |
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RFC 3891 The SIP "Replaces" Header September 2004
present in an incoming request. In other words, the to-tag parameter
is compared to the local tag, and the from-tag parameter is compared
to the remote tag.
If more than one Replaces header field is present in an INVITE, or if
a Replaces header field is present in a request other than INVITE,
the UAS MUST reject the request with a 400 Bad Request response.
The Replaces header has specific call control semantics. If both a
Replaces header field and another header field with contradictory
semantics are present in a request, the request MUST be rejected with
a 400 "Bad Request" response.
If the Replaces header field matches more than one dialog, the UA
MUST act as if no match is found.
If no match is found, the UAS rejects the INVITE and returns a 481
Call/Transaction Does Not Exist response. Likewise, if the Replaces
header field matches a dialog which was not created with an INVITE,
the UAS MUST reject the request with a 481 response.
If the Replaces header field matches a dialog which has already
terminated, the UA SHOULD decline the request with a 603 Declined
response. (If the matched invitation was just terminated, the
replacement request should fail as well. Declining the request with
a 600-class response prevents an irritating race-condition where the
UA rings or alerts for a replacement call which is not wanted.)
If the Replaces header field matches an active dialog, the UA MUST
verify that the initiator of the new INVITE is authorized to replace
the matched dialog. If the initiator of the new INVITE has been
successfully authenticated as equivalent to the user who is being
replaced, then the replacement is authorized. For example, if the
user being replaced and the initiator of the replacement dialog share
the same credentials for Digest authentication [6], or they sign the
replacement request with S/MIME [7] with the same private key and
present the (same) corresponding certificate used in the original
dialog, then the replacement is authorized.
Alternatively, the Referred-By mechanism [4] defines a mechanism that
the UAS can use to verify that a replacement request was sent on
behalf of the other participant in the matched dialog (in this case,
triggered by a REFER request). If the replacement request contains a
Referred-By header that corresponds to the user being replaced, the
UA SHOULD treat the replacement as if the replacement was authorized
by the replaced party. The Referred-By header SHOULD reference a
corresponding, valid Refererred-By Authenticated Identity Body [5].
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RFC 3891 The SIP "Replaces" Header September 2004
The UA MAY apply other local policy to authorize the remainder of the
request. In other words, the UAS may apply a different policy to the
replacement dialog than was applied to the replaced dialog.
In addition, the UA MAY use other authorization mechanisms defined
for this purpose in standards track extensions. Extensions could
define other mechanisms for transitively asserting authorization of a
replacement.
If authorization is successful, the UA attempts to accept the new
INVITE, reassign the user interface and other resources of the
matched dialog to the new INVITE, and shut down the replaced dialog.
If the UA cannot accept the new INVITE (for example: it cannot
establish required QoS or keying, or it has incompatible media), the
UA MUST return an appropriate error response and MUST leave the
matched dialog unchanged.
If the Replaces header field matches a confirmed dialog, it checks
for the presence of the "early-only" flag in the Replaces header
field. (This flag allows the UAC to prevent a potentially
undesirable race condition described in Section 7.1.) If the flag is
present, the UA rejects the request with a 486 Busy response.
Otherwise, it accepts the new INVITE by sending a 200-class response,
and shuts down the replaced dialog by sending a BYE. If the Replaces
header field matches an early dialog that was initiated by the UA, it
accepts the new INVITE by sending a 200-class response, and shuts
down the replaced dialog by sending a CANCEL.
If the Replaces header field matches an early dialog that was not
initiated by this UA, it returns a 481 (Call/Transaction Does Not
Exist) response to the new INVITE, and leaves the matched dialog
unchanged. Note that since Replaces matches only a single dialog,
the replacement dialog will not be retargeted according to the same
forking logic as the original request which created the early dialog.
(Currently, no use cases have been identified for replacing just a
single dialog in this circumstance.)
4. User Agent Client Behavior: Sending a Replaces Header
A User Agent that wishes to replace a single existing early or
confirmed dialog with a new dialog of its own, MAY send the target
User Agent an INVITE request containing a Replaces header field. The
User Agent Client (UAC) places the Call-ID, to-tag, and from-tag
information for the target dialog in a single Replaces header field
and sends the new INVITE to the target. If the user agent only
wishes to replace an early dialog (as in the Call Pickup example in
Section 7.1), the UAC MAY also include the "early-only" parameter in
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RFC 3891 The SIP "Replaces" Header September 2004
the Replaces header field. A UAC MUST NOT send an INVITE with a
Replaces header field that attempts to replace an early dialog which
was not originated by the target of the INVITE with a Replaces header
field.
Note that use of this mechanism does not provide a way to match
multiple dialogs, nor does it provide a way to match an entire call,
an entire transaction, or to follow a chain of proxy forking logic.
For example, if Alice replaces Cathy in an early dialog with Bob, but
Bob does not answer, Alice's replacement request will not match other
dialogs to which Bob's UA redirects, nor other branches to which his
proxy forwards. Although this specification takes reasonable
precautions to prevent unexpected behavior in the face of forking,
implementations SHOULD only address replacement requests (i.e., set
the Request-URI of the replacement request) to the SIP Contact URI of
the target.
5. Proxy behavior
Proxy Servers do not require any new behavior to support this
extension. They simply pass the Replaces header field transparently
as described in the SIP specification.
Note that it is possible for a proxy (especially when forking based
on some application layer logic, such as caller screening or time-
of-day routing) to forward an INVITE request containing a Replaces
header field to a completely orthogonal set of Contacts other than
the original request it was intended to replace. In this case, the
INVITE request with the Replaces header field will fail.
6. Syntax
6.1. The Replaces Header
The Replaces header field indicates that a single dialog identified
by the header field is to be shut down and logically replaced by the
incoming INVITE in which it is contained. It is a request header
only, and defined only for INVITE requests. The Replaces header
field MAY be encrypted as part of end-to-end encryption. Only a
single Replaces header field value may be present in a SIP request.
This document adds the following entry to Table 2 of [1]. Additions
to this table are also provided for extension methods defined at the
time of publication of this document. This is provided as a courtesy
to the reader and is not normative in any way. MESSAGE, SUBSCRIBE
and NOTIFY, REFER, INFO, UPDATE, PRACK, and PUBLISH are defined
respectively in [15], [16], [8], [17], [18], [19], and [20].
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RFC 3891 The SIP "Replaces" Header September 2004
Message *7: Bob's desk -> Alice
SIP/2.0 487 Request Terminated
To: <sip:bob@example.org>;tag=6472
From: <sip:alice@example.org>;tag=7743
Call-ID: 425928@phone.example.org
CSeq: 1 INVITE
8. Security Considerations
The extension specified in this document significantly changes the
relative security of SIP devices. Currently in SIP, even if an
eavesdropper learns the Call-ID, To, and From headers of a dialog,
they cannot easily modify or destroy that dialog if Digest
authentication or end-to-end message integrity are used.
This extension can be used to disconnect participants or replace
participants in a multimedia conversation. As such, invitations with
the Replaces header MUST only be accepted if the peer requesting
replacement has been properly authenticated using a standard SIP
mechanism (Digest or S/MIME), and authorized to request a replacement
of the target dialog. All SIP implementations are already required
to support Digest Authentication. In addition, implementations which
support the Replaces header SHOULD also implement the Referred-By
mechanism.
How a User Agent determines which requests are legitimately
authorized to make dialog replacements is non-trivial and depends on
a considerable amount of local policy configuration. In general,
there are four cases when an authorization for a replacement is
reasonable or warranted.
1. Replacement made by a party considered equivalent to the replaced
party
2. Replacement made on behalf of the replaced party (perhaps
transitively)
3. Replacement made by a former participant
4. Replacement made by a specifically authorized party
Starting with #1 for example, if an executive and an assistant both
receive requests for a shared address-of-record, if so configured,
either should be able to replace dialogs of the other for the shared
identity. Both could even share the same keying material (Digest or
S/MIME), or one could hold an authorization document signed by the
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RFC 3891 The SIP "Replaces" Header September 2004
other expressing this relationship. Likewise, in a call center
environment, each call center agent could possess credentials to
which supervisors also have access.
The most common use case of a replacement is on the request of the
replaced participant (who no longer wants to be involved). This is
the case in many features, such as completing an Attended Transfer
and converting a 3-way call to a point-to-point call. Such
replacements are typically triggered by a REFER [8] request from the
replaced participant. The Referred-By [4] mechanism defines one way
to identify the apparent original requester and can point to a SIP
Authenticated Identity Body [5] (an S/MIME-based signed assertion) to
secure this information.
In the example in section 1, Alice sends an INVITE with Replaces to
Bob. Alice was a former participant in the conversation and had a
previous dialog relationship with Bob. Alice can use the same Digest
or S/MIME credentials she used to authenticate with Bob during the
original call to prove that she was a former participant. Note that
this justification for replacing calls is more dangerous than the
others, and in most cases is another way to authorize that the
replacing participant is available. Implementations SHOULD NOT rely
on this method as an authorization mechanism.
The last scenario is the easiest to secure but the least likely to be
useful in practice. It is unlikely that an arbitrary host in the
Internet is aware of any special authorization relationship between
the replaced and the replacing parties. However, this use case may
be useful in some environments. Since this usage does not
effectively degrade the security of the solution, it is still
allowed.
Some mechanisms for obtaining the dialog information needed by the
Replaces header (Call-ID, to-tag, and from-tag) include URIs on a web
page, subscriptions to an appropriate event package, and
notifications after a REFER request. Since manipulating this dialog
information could cause User Agents to replace the wrong dialog, use
of message integrity protection for this information is STRONGLY
RECOMMENDED. Use of end-to-end security mechanisms to encrypt this
information is also RECOMMENDED.
This extension was designed to take advantage of future signature or
authorization schemes defined in standards track extensions. In
general, call control features benefit considerably from such work.
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RFC 3891 The SIP "Replaces" Header September 2004
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